Runic.Navigation 0.7.0-preview.4

Runic.Navigation

Typed, host-neutral navigation for .NET ViewModels. RunicNavigator owns navigation regions with stable entry ids, awaited departure guards, initialize-once and resume-on-return hooks, results from an entry, and owned or borrowed content. IRunicModelContext serializes the ViewModel changes that navigation commits. The types are in the Runic.Navigation namespace.

The package has no build targets, generator, Node or UI host, and it supports trimming and NativeAOT. Its only dependencies are Microsoft.Extensions.DependencyInjection.Abstractions and Microsoft.Extensions.Logging.Abstractions. Runic.Application depends on it and presents regions in web Views.

Install

dotnet add package Runic.Navigation --prerelease

Runic.Application apps already have it through that package.

Experimental API

RunicNavigator owns typed navigation regions. Each region gives its entries stable ids, and it supports awaited departure guards, initialize-once and resume-on-return hooks, and owned or borrowed content. The navigation API is experimental: every type is marked [Experimental("RUNICNAV001")]. Suppress RUNICNAV001 to use it, and expect changes before it is supported. The model context types are not experimental. The navigator replaces ReactiveUI's RoutingState; it does not wrap one. See the ReactiveUI adapter for observables and a back command. WPF apps present regions with Runic.Navigation.Wpf.

using Runic.Navigation;

services.AddRunicNavigation(); // one model context and navigator per window scope

var main = navigator.CreateRegion<IMainViewModel>(this, NavigationTarget.Borrow<IMainViewModel>(home));
// The navigator constructs the document from its services, and owns it.
var result = await main.PushAsync<DocumentViewModel>();
if (result is NavigationResult<IMainViewModel>.Committed) { /* main.Current is the document */ }
await main.BackAsync(); // the document retires and is disposed; home resumes

AddRunicNavigation() registers a scoped IRunicModelContext and a scoped RunicNavigator that uses it. Dispose window scopes with DisposeAsync (CreateAsyncScope). The navigator is also IDisposable for containers that only dispose synchronously: Dispose starts the same shutdown and returns without waiting for it. Without DI, construct the navigator with RunicNavigatorOptions (ModelContext, Services, CreateEntryScopes, LoggerFactory, TimeProvider, CloseTimeout), and use the same model context for the window's session.

Operations and results

PushAsync, ReplaceAsync and ResetAsync take a target, or a ViewModel type that the navigator constructs: PushAsync<DocumentViewModel>(), or PushAsync<DocumentViewModel, NoteId>(id) for a ViewModel initialized with input. PushForResult<ConfirmViewModel, bool>() does the same for a result. See Container-resolved targets.

• Every operation returns a NavigationResult<TContent>. The outcomes are returned, never thrown:

  • Committed, with the new current entry and the retired entries.
  • Rejected, with a Reason: Guard, Cancelled, Closed, Reentrant, NoHistory, NotCurrent or EntryNotFound.
  • Failed, with the Phase (Guarding, Preparing or Committing) and the exception (Error).
  • Superseded.

  Operations throw only for argument and ownership errors.

• NavigationRequestOptions.ExpectedCurrent makes a request conditional. It is rejected as NotCurrent if that entry is not current at admission or at commit. Use it, or NavigationEntryContext.BackAsync, for work that completes late, such as a double click or a dialog that answers after the user moved on.

• A region exposes Current, CurrentEntry, History, CanGoBack and IsTransitioning. It raises PropertyChanged for them inside a model turn, the commit turn for a navigation.

Entry lifecycle

An entry starts Pending and becomes Active when its push commits. It then moves between Active and Retained as pushes and Backs commit, and ends Retired. Every state can retire:

Pending ──commit──▶ Active ◀──Push / Back(To)──▶ Retained
   │                  │                              │
   │ not committed    │ Back, Replace,               │ BackTo, Reset,
   │                  │ Reset, Clear                 │ ClearHistory, Clear
   ▼                  ▼                              ▼
                    Retired

A closing parent region or navigator disposal also retires entries in any state.

Retirement runs once per entry, whichever path reaches it first: commit cleanup, a closing parent, a failed preparation or navigator disposal. It runs these steps in order, and each step runs even if an earlier one failed:

1. NavigationEntryContext.Retirement is cancelled.
2. The child regions that the entry's content owns close. Their in-flight transitions end as Rejected(Closed), and their entries retire depth-first.
3. Owned content only: its presentations are forgotten.
4. Owned content only: the content is disposed, outside model turns. Then the entry's service scope, if the navigator created one, is disposed.
5. Owned content only: its model-context lease is released.

A failed step is logged as event 1064.

Transitions

• A plain BackAsync() (no cancellable token) that arrives while the region's latest request is another plain Back that hasn't committed, and neither the region nor the child regions the departing entry owns have changed since that Back was admitted, joins it instead of superseding it: both callers get the same result, the departure guard runs once, and a confirm dialog stays open (event 1074). Back from code while the guard asks therefore pops once and asks once. A Back that starts after the first one committed is a new request and pops again; that is stack semantics. A Back with a cancellable token never joins and is never joined, so cancelling it never cancels another caller's Back; it supersedes as above. A return from a result entry (CompleteAsync, DismissAsync) is never joined either.
• IsTransitioning is true from admission, synchronously. A command that reads it in CanExecute, such as NavigationHost's BrowseBack in Runic.Navigation.Wpf, is disabled before the Back call returns, so a double click pops once.
• Known limitation: the ReactiveUI adapter's Back command passes a cancellable token and updates CanExecute through its output scheduler, so a second Back through it while a guard asks supersedes the first: the first confirm closes and a second one opens. History and edits stay intact. Joining such Backs is tracked in #147.
• A dialog answers its guard with CompleteAsync or DismissAsync. Code run from the confirm dialog that awaits BackAsync() on the guarded region before answering joins the Back that waits for that answer, so it doesn't complete until the dialog closes another way.
• A request is superseded only before it commits, never after. A cancelled token gives Rejected(Cancelled) and navigator disposal gives Rejected(Closed), in any phase before the commit.
• Hook tokens are cancelled when the request is superseded, cancelled or closed. A superseded request that is still running 5 seconds later is logged once (event 1067). The next request waits for it.
• Admission inside a model turn does not block. The transition continues on the thread pool, so no hook runs in the caller's turn. Don't block on its result in that turn.
• Admission is per region, so a guard in one region may await a request in a sibling region, as the confirm dialog below does. Parent and child regions coordinate through the plan: a child commit during the parent's guard supersedes the parent's request.
• One entry's guard never runs twice at the same time. It can still run again for the same departure: a parent transition that retires a child region asks the child's guards even when a child transition already asked them. Write guards to be repeatable. A LeaveConfirmation keeps its yes while the transition it allowed is unsettled and passes it to the transition that supersedes it, so the rerun doesn't ask again. When a change in another region, rather than a later request, superseded the transition, the yes ends, and the user may be asked twice.

Ownership

• Content resolved from a container (a window scope, the root provider or Splat) belongs to that container, so Borrow it. Use Own and Create only for content constructed with new or by the navigator. A factory receives the entry's IServiceProvider to resolve constructor dependencies. Those dependencies stay container-owned. Without entry scopes, don't resolve disposable transients for it: the provider keeps them until it is disposed.
• An instance can be owned once. Own of an instance the navigator has already owned, whether live or retired, throws InvalidOperationException. A Create factory that returns one gives Failed(Preparing).
• Present owned content only through its region's slot. Retirement forgets every presentation of the instance.
• Owned content is disposed outside model turns, on the model's thread when the context schedules hooks (see Threading). A Dispose that touches shared model state uses ModelContext.InvokeAsync, and it must not await navigation on the same navigator.
• A region's owner can be any object. Regions owned by an owned entry's content are that entry's children, and they close when it retires. NavigationRegionOptions.WhileParentRetained decides what a child region does while its parent entry is retained: Keep (the default), ResetToRoot or Clear. Regions owned by borrowed content or by the root close only with the navigator.
• An initial target (CreateRegion(owner, initial)) becomes current without a transition. Its content must not implement INavigationInitialize or INavigationInitialize<TInput>, and Create(factory, input) and Create<T, TInput>(input) cannot be initial targets.

Container-resolved targets

NavigationTarget.Create<T>() and Create<T, TInput>(input) let the navigator construct the ViewModel with ActivatorUtilities.CreateInstance from the entry's service provider. The constructor's parameters come from the container; the ViewModel itself is never resolved, so the navigator owns it. The region methods PushAsync<T>(), PushAsync<T, TInput>(input), the same forms of ReplaceAsync and ResetAsync, and PushForResult<T, TResult>() wrap these targets. T must be a TContent, checked at compile time.

await main.PushAsync<DocumentViewModel>();                      // DocumentViewModel(EditorViewModel editor, ...)
await main.PushAsync<NoteViewModel, NoteId>(id);                // NoteViewModel : INavigationInitialize<NoteId>
var confirm = dialog.PushForResult<ConfirmViewModel, bool>();
var picked = dialog.PushForResult<bool>(NavigationTarget.Create<ConfirmViewModel, string>("Discard?")); // input and a result

• Constructors are kept for trimming and NativeAOT ([DynamicallyAccessedMembers(PublicConstructors)] on T). [ActivatorUtilitiesConstructor] picks one of several constructors.
• A navigator without Services uses an empty provider. A constructor that needs services then fails as Failed(Preparing) (event 1061), like a throwing factory.
• INavigationEntry.Services and NavigationEntryContext.Services return the provider that built the entry. After an entry with its own scope retires, resolving from it throws ObjectDisposedException.

Entry scopes. With RunicNavigatorOptions.CreateEntryScopes, the navigator creates one IServiceScope for each entry that a Create target builds, initial targets included. The factory and ActivatorUtilities resolve from it, and the entry's Services returns it. Retirement disposes the content, then the scope, so scoped and disposable transient dependencies live exactly as long as the entry. A construction that fails disposes the scope at once. Borrow and Own entries never get a scope.

Turn entry scopes on only for a navigator whose Services is the root provider, for example a singleton navigator. A navigator resolved from a scope, such as the one AddRunicNavigation() registers, must leave it off: CreateScope on a scoped provider makes a sibling scope of the root, so the entry's scoped dependencies would silently differ from the navigator's. The navigator can't tell the two providers apart. AddRunicNavigation() leaves it off; a window scope already bounds those dependencies.

Presenting regions

A host presents a region's Current. In Runic.Application, a get-only NavigationRegion<TContent> property of a ViewModel is a content slot that the generated Bridge presents in the window's web View; see Content slots. Other hosts bind INavigationRegion, the non-generic view of a region, and INavigationEntry. Runic.Navigation.Wpf provides WPF hosts and a dispatcher model context. RunicNavigator.AttachPresentation is for presentation integrations only: retirement calls INavigationPresentation.Forget once for each attached presentation and owned entry.

Results from an entry

PushForResult<TResult>(target) pushes an entry and returns a NavigationResultRequest:

• Transition is the push's result.
• Completion ends Completed(value) only when the entry calls NavigationEntryContext.CompleteAsync(value) and the Back that it issues commits.
• Completion ends Dismissed on every other path: the push does not commit, the entry retires another way, the caller's token is cancelled after the commit (which also goes back from the entry), or the navigator closes.

A Back from CompleteAsync or from caller cancellation may leave the region empty, so a dialog region can start empty. A push onto an entry whose caller cancelled retires that entry instead of retaining it. That Back can still be rejected, for example when a push over the entry is already committing or the entry is no longer on top. Then the dismissed entry stays in the history: a later Back resumes it, and its CompleteAsync goes back, possibly emptying the region, and drops the value (event 1071).

A rejected CompleteAsync, for example one vetoed by a guard, leaves the request open, and the entry can try again. CompleteAsync accepts any value of the request's result type at run time, so true completes a bool? request and 5 an object request. Completions run their continuations asynchronously, after the commit turn. A completion is set when the entry starts retiring, so it can be observed while the entry's owned content is still being disposed.

NavigationEntryContext.DismissAsync() leaves an entry without a result. For the current entry it ends an open request Dismissed at once and goes back, which may leave the region empty, like a cancelled caller token. Called from the entry's own InitializeAsync, it cancels the push that creates the entry (Rejected(Cancelled)) and returns at once; don't await that push from the hook. For a retained entry it ends the request and leaves the entry in place; for a retired one it does nothing. Both return Rejected(NotCurrent). A dialog's Cancel uses it, so Cancel always ends the question, even when its Back is rejected because something was pushed over the dialog.

Leaving with unsaved changes

A guard's true is not a commit: a later request can still supersede the departure, another guard can veto it, or it can fail. Work that must happen only when the departure happens goes to NavigationDeparture.OnCommitted:

public async ValueTask<bool> CanDepartAsync(NavigationDeparture departure, CancellationToken token)
{
    if (!await departure.ModelContext.InvokeAsync(() => Editor.IsDirty, token)) return true; // guards run outside turns
    if (!await AskAsync(token)) return false;
    departure.OnCommitted(Editor.DiscardChanges);
    return true;
}

• Actions run in the commit turn of the departure, in registration order, after the new navigation state is applied and before the regions raise PropertyChanged. They don't run when the transition is superseded, rejected, cancelled or fails.
• Register while the guard runs. OnCommitted throws InvalidOperationException after the guard returned.
• An action that throws is logged (event 1072); later actions still run, and the commit stands. Actions must not pump or await navigation.

LeaveConfirmation packages the whole pattern. It allows a departure that only retains the entry (unless askOnRetain), allows when there are no unsaved changes, asks otherwise, and discards with OnCommitted. With askOnRetain, a confirmed push over the entry discards too, although the entry stays in the history. Its yes stands while the departure it allowed is unsettled: when the guard reruns for the same entry, for a parent transition or a request that superseded the one it allowed, it answers without asking again, and discard still runs at most once. LeaveConfirmation.InDialog asks with a dialog pushed for a bool result into another region; only Completed(true) confirms, and the guard's token dismisses the dialog when the departure is superseded or the navigator closes. The dialog region must not be the guarded region, one of its ancestors or one of its descendants: that push would be Rejected(Reentrant).

LeaveConfirmation keeps the caller's context, so its confirm delegate stays on the UI thread. Without a hook scheduler the guard can run in the frame of the call that started the navigation. If that call blocks a UI thread on the navigation's result (for example with GetAwaiter().GetResult()), the guard's continuation can never run there, and the two deadlock. Await navigation results; don't block on them, as with any asynchronous code.

The Notes example asks this way:

// The document: the guard forwards to a LeaveConfirmation.
_leave = LeaveConfirmation.InDialog(navigation.Dialog,
    () => NavigationTarget.Own<IDialogViewModel>(new ConfirmNavigationViewModel("Discard the unsaved edits?")),
    () => Editor.IsDirty, Editor.DiscardChanges);

public ValueTask<bool> CanDepartAsync(NavigationDeparture departure, CancellationToken token) =>
    _leave.CanDepartAsync(departure, token);

// The confirm captures its entry in InitializeAsync and answers with it.
public ValueTask InitializeAsync(NavigationEntryContext entry, CancellationToken token)
{ _entry = entry; return ValueTask.CompletedTask; }
private Task Confirm() => _entry.CompleteAsync(true).AsTask();
private Task Cancel() => _entry.DismissAsync().AsTask();

Window close and disposal

DI disposes the navigator before the model context it depends on. Its DisposeAsync:

1. Refuses new requests and dismisses every open PushForResult request, so guards that await a result unblock.
2. Cancels in-flight transitions and waits for them, and for running cleanup, up to CloseTimeout (10 seconds on its TimeProvider). A late transition never commits.
3. Retires every remaining entry, region by region, pending entries included. Clearing a closing region normally runs in a model turn. These turns share one CloseTimeout, so disposal takes about twice CloseTimeout at most, plus owned content disposal. If a turn does not run in time, the stack is cleared outside a turn (event 1068).

Window close does not run guards. Disposing a navigator cancels window-owned operations that its owned content started, and then disposes that content.

WhenIdleAsync() waits until no transition or retirement is running.

Threading

Without a scheduler, as with RunicModelContext, hooks (factories, guards, initialize and resume) run outside turns on the thread that runs the transition, and owned content is disposed there too. A model context whose model has thread affinity, such as a UI dispatcher context, can also implement IRunicModelHookScheduler. The navigator then runs each hook and each owned content disposal as its own operation on the model's thread: never inline in the caller, never inside a turn. Hook operations don't count as turns. Once the context is closed, disposal runs on the thread pool. A decorator that wraps a scheduling context must implement and forward the interface too.

A context that can close on its own, for example when its UI thread shuts down, can also implement IRunicModelContextLifetime. The navigator subscribes to its Closed token and starts closing when it fires: later requests are Rejected(Closed) at once, and transitions in flight are cancelled as closed. Without it, the navigator learns about the close only when a turn or hook fails with ObjectDisposedException, so a request can wait behind a transition whose hook ignores its cancellation. Decorators forward this interface too.

During DisposeAsync, a scheduled disposal that hasn't started by the clearing turns' deadline runs on the thread pool instead, exactly once, so disposal completes even when the model's thread is blocked. Don't block the model's thread on DisposeAsync (for example GetAwaiter().GetResult() in a WPF OnExit): call Dispose() there, which starts disposal without waiting. A cleanup that starts after disposal finished (for example content that a factory returns late) waits on the model's thread without a bound. If that thread never runs it again and the context never closes, that content and its model-context lease are never released. Close the context when the application shuts down, for example on Dispatcher.ShutdownStarted in WPF.

The navigator itself never continues on the model's thread: after awaiting a hook, a turn or other user code, it moves to the thread pool when the completion ran in the model context. A caller that awaits a navigation result resumes on its own context as usual.

Pitfalls

• A hook that disposes its own navigator stalls disposal for CloseTimeout, because disposal waits for the transition that runs the hook.
• A hook that awaits WhenIdleAsync() deadlocks: the navigator is not idle while that hook runs.
• A child guard that commits a change in its own region supersedes the parent's transition, because the parent's commit sees the child's version change.
• A request from a hook into the same region, one of its descendants or one of its ancestors is Rejected(Reentrant). For example, a parent's InitializeAsync cannot push into its own child region, and a CurrentPane guard cannot call Main.BackAsync() on the region that holds its document. Issue the request after the transition commits. A hook's code is the hook's own frame and its continuations, including await Task.Yield() and awaits that resume through the SynchronizationContext the hook started with: requests from there stay Rejected(Reentrant). Input handlers that a hook's nested message loop dispatches, for example while a guard shows a modal window, are not the hook. One gap fails safe: a handler that the nested loop dispatched and that awaits past the loop's end, while the transition is still in flight, may continue as the hook and get Rejected(Reentrant).
• A service that creates a region with a container-built initial target whose constructor needs that service is a dependency cycle. For example, MainNavigation creates its region with Create<HomeViewModel>(), and HomeViewModel(MainNavigation navigation) resolves MainNavigation again, which creates another region. CreateRegion detects the re-entry for the same owner type and initial target type on the same thread, on any navigator, and throws InvalidOperationException instead of overflowing the stack. One holder type reused at two levels with different targets is fine. Create the region empty and reset it after construction: await navigation.Region.ResetAsync<HomeViewModel>(), for example from application startup.

Model context

IRunicModelContext runs short synchronous turns one at a time, in posting order. Navigation commits run as turns, and region PropertyChanged handlers run inside them. InvokeAsync runs a turn and returns its result; called from a turn of the same context, it runs inline. TryPost queues a turn without waiting. RunicModelContext is the default implementation, and AddRunicNavigation() registers one per scope unless one is registered.

RunicModelContextRegistry associates each model object with the context that owns its changes. Bind registers objects with an application-owned context. Acquire creates a context for a model graph, reuses it for later leases of the same graph, and disposes it after the last lease is released. A second context for an object that is already bound is rejected.

Disposing a RunicModelContext rejects queued turns and waits for the running turn. A posted turn that disposal drops is logged (event 1031).

Testing

RunicNavigator.UnretiredEntryCount counts entries that have not finished retiring. After DisposeAsync, it is 0 when every entry retired.

Logging

Errors and warnings carry the exception. Rejections, supersessions, joined Backs and dismissed results log at Debug. The properties are Region (the TContent type name), RegionId (a per-navigator number that tells apart regions with the same TContent), Operation, EntryType, Step, Reason, Property (event 1063, the region property whose handler threw) and ErrorType, never values.

• Events 1060-1074 use the category Runic.Navigation (RunicNavigator.LogCategory), from RunicNavigatorOptions.LoggerFactory or the scope's ILoggerFactory with AddRunicNavigation().
• Events 1030-1033 use the logger of the RunicModelContext: ILogger<RunicModelContext> (category Runic.Navigation.RunicModelContext) when DI or a Runic.Application window session with a logger factory created it. Event 1033 is also logged by the registry when a context it created fails to shut down.
• Without a logger factory, each entry is written to System.Diagnostics.Trace (TraceError, or TraceWarning for warnings) as its formatted message followed by the exception.

Navigation reserves events 1060-1079: 1060-1069 for transitions and cleanup, and 1070-1079 for results and later navigation events, of which 1075-1079 are not used yet. Events 1043-1049 are reserved for the ReactiveUI navigation adapter.